This invention relates to a method and apparatus for measuring and monitoring the axial load on and the axial position of a valve stem used to open and close a valve. It relates particularly to a method and apparatus for measuring and monitoring from a remote location, the axial load on and the axial position of a valve stem used in an air operated valve. This invention is especially useful in the field of valve diagnostics.
In the nuclear power industry, valves are actuated remotely by electric motors or compressed air to open, closed or intermediate positions to improve or maintain power plant output and to maintain proper cooling of the nuclear reactor. Continuous and proper operation of the valves is essential for reliable operation of the power plant and the safety of the general public. The power and process industries and the valve manufacturing industry have had a need to develop apparatus and systems to continuously measure, test and monitor various operating parameters associated with valves, especially remotely located valves, and to alert the plant operator of any significant changes in any of the operating parameters being measured, tested and monitored.
U.S. Pat. No. 4,542,649 to Charbonneau et al. discloses a recently developed motor operated valve monitoring system that measures, records and correlates a number of valve operating parameters, such as valve stem load, valve stem movements, valve stem torque, spring pack movement and motor current. The information provided by the system described in the Charbonneau et al. patent provides a direct indication to the operator of any valve problems, such as excessive or inadequate packing loads, excessive inertia, incorrectly set limit and torque switches, malfunctioning thermal overload devices, excessive stem thrust loads, gear train wear and damage to the valve stem.
As a result of the teachings of the Charbonneau et al. patent, efforts have been made by Movats, Inc., the assignee of the Charbonneau et al. patent, as well as the assignee of the present patent application, to develop additional apparatus and systems by which the operation and performance of different types of valves could be easily measured, tested and monitored by an operator at a location adjacent to or even remote from the valve. U.S. Pat. Nos. 4,787,245; 4,888,996; 4,912,984; 4,930,228; 5,000,040 and 5,142,906 describe several of the valve monitoring and diagnostic systems developed by Movats, Inc. U.S. Pat. Nos. 4,570,903; 4,805,451 and 4,856,327 describe several valve monitoring and diagnostic systems developed by others.
Two of the important operating parameters to be measured, tested and monitored in a valve are the axial load or thrust exerted on the valve stem by the valve operating motive power, such as an electric motor or compressed air, and the axial position of the valve stem, which is also indicative of the position of the valve. The valve monitoring and diagnostic systems described in the above-noted prior patents have been developed primarily for valves operated by an electric motor and are not easily adapted to valves operated by compressed air.
The above-noted U.S. Pat. No. 4,542,649 discloses a compressive load cell mounted on the top of the motor operated valve actuator housing to measure valve stem load or thrust as the valve stem moves axially in the valve opening direction. The above-noted U.S. Pat. No. 4,570,903 discloses a tension load cell to measure valve stem load or thrust as the valve stem moves axially in a valve closing direction. The above-noted U.S. Pat. No. 4,805,451 discloses attaching a strain sensing device to the exterior of the valve yoke in an attempt to measure the axial stresses in the valve yoke, instead of the valve stem itself. The above-noted U.S. Pat. No. 4,856,327 discloses load cells, clamped under a compressive preload between the valve actuator and the valve yoke in an attempt to measure tensile and compressive reaction forces which are claimed to be proportional to actual valve stem load or thrust.
The systems that have been developed in the prior art for measuring valve stem axial load or thrust, in addition to being primarily adapted for motor operated, rather than air operated valves, usually measure the valve stem axial load or thrust indirectly and therefore must be carefully calibrated for each installation and then regularly recalibrated during service to determine how the measurements relate to the actual valve stem axial load or thrust. In addition, the systems developed in the prior art for measuring and monitoring valve stem axial load or thrust are not able to measure the axial position of the valve stem, which is indicative of the position of the valve